Quantum dot integration in heterostructure solar cells

InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm² light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm², shows an open circuit voltage V_oc of 0.7 V, a short circuit current I_sc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).     

Improving the optical efficiency and concentration of a single-plate quantum dot solar concentrator using near infra-red emitting quantum dots

Low luminescent quantum yields and large overlap between quantum dot (QD) emission and absorption spectra of present commercially-available visible-emitting QDs have led to low optical efficiencies for single-plate quantum dot solar concentrators (QDSCs). It is shown that using near infra-red (NIR) emitting QDs, re-absorption of QD emitted photons can be reduced greatly, thereby diminishing escape cone losses thus improving optical efficiencies and concentration ratios. Using Monte-Carlo ray-trace modelling, escape cone losses are quantified for different types of QD. A minimum 25% escape cone loss would be expected for a plate with refractive index of 1.5 containing QDs with no spectral overlap. It is shown that escape cone losses account for ∼57% of incident photons absorbed in QDSCs containing commercially-available visible-emitting QDs.     

Quantum dot containing nanocomposite thin films for photoluminescent solar concentrators

Silicon oxide films containing CdS quantum dots have been deposited on glass substrates by a sol–gel dip-coating process. Hereby the CdS nanocrystals are grown during the thermal annealing step following the dip-coating procedure. Total hemispherical transmittance and reflectance measurements were carried out by means of a spectrophotometer coupled to an integrating sphere. For CdS-rich films, an absorption edge at photon energies in the vicinity of the band gap value of bulk CdS is observed. For lower CdS concentrations, the absorption edge shifts to higher photon energies, as expected for increasing quantum confinement. The samples show visible photoluminescence which is concentrated by total internal reflection and emitted at the edges of the substrate. The edge emission has been characterized by angle-dependent photoluminescent (PL) spectroscopy. Information on the lateral energy transport within the sample can be extracted from spectra obtained under spatial variation of the spot of excitation. The color of the photoluminescence can be tuned by varying the annealing temperature which governs crystal growth and thus the cluster size distribution. The characteristic features observed in the PL spectra clearly exhibit a blueshift for lower annealing temperatures, confirming the presence of quantum size effects.Advantages of the proposed concept of quantum dot containing coatings on glass panes for photoluminescent solar concentrators are the high potential for low-cost fabrication on the large scale and the suitability for architectural integration.     

利用半导体量子点的高效太阳电池

1前言 第三代太阳电池的开发 太阳电池，主要是由做在半导体基片上的pn结组成。如图1所示若在pn结处射入太阳光，则在半导体内部激发出电子-空穴对，在内部电场作用下，电子向n侧。空穴向p侧迅速流动，产生光生电功率。现在广泛使用的是厚度200～350μm的单晶硅，及用铸造法制作的多晶硅太阳电池。最近为了降低硅太阳电池的制造成本，开发了薄膜太阳电池。在薄膜太阳电池的情形下由于光吸收层的厚度仅为0．2～3μm左右，使用的材料大大减少。现在还在开发非晶硅、微结晶硅、CdTe、Cu（InGa）、Se2（CIGS）等太阳电池。     

Pasteurization of goat milk using a low cost solar concentrator

This paper presents the design of a system for pasteurizing goat milk as a part of a process for artisan elaboration of cheese. The system consists in a Fresnel type concentrator used for cooking large amounts of food with a vaporizer located in the focus. The steam bubbles into the isolated container where the milk is cooked by a double boiler.When the desired temperature is reached, the steam flow is closed and the milk remains in this condition for 30 min into the closed container. Pasteurizing of 10 l of milk is done in about 1 h. The project considers guidelines for achieving a successful process.     

Solar distillation of water using a V-trough solar concentrator with a wick-type solar still

A V-trough solar concentrator has been combined with an inclined flat-plate wick-type solar still. Outdoor testing was carried out with and without the solar concentrator on clear days in summer and winter. The equipment was used to investigate the enhancement of the outdoor performance of the wick-type solar still by the solar concentrator.It has been concluded that use of the solar concentrator with the inclined wick-type solar still can lead to a greater fractional increase in still efficiency and productivity on clear days in winter than on clear days in summer.     

To make polymer: Quantum dot hybrid solar cells NIR-active by increasing diameter of PbSnanoparticles

We fabricate NIR-active solar cells based on PbS quantum dots and a conventional conjugated polymer. These devices act as solar cells under exclusively NIR wavelengths above 650 nm. Here PbS nanoparticles absorb photons in the NIR range that in turn generate excitons. We show that with an assistance from a strong electron-acceptor (TiO₂), these excitons can be dissociated to electrons and holes to yield a photocurrent in the external circuit. We then aim to extend the spectral window of the solar cells to higher wavelength region by increasing the diameter of PbS nanoparticles to make the cells further NIR-active. We observe that the short-circuit current (J_SC) shows a peak when the diameter of PbS nanoparticles increases. Here, the spectral window can be extended till conduction band-edge of PbS quantum dots falls below that of TiO₂ nanostructures cutting off the electron-transfer pathway. The NIR-active photovoltaic solar cells yield a short-circuit current (J_SC) of 1.0 mA/cm², open-circuit voltage (V_OC) of 0.42 V, and power conversion efficiency (η) of 0.16% and remain operative till 1200 nm.     

Quantum dot solar concentrators: Electrical conversion efficiencies and comparative concentrating factors of fabricated devices

A novel, non-tracking concentrator is described, which uses nano-scale quantum dot technology to render the concept of a fluorescent dye solar concentrator (FSC) a practical proposition. The quantum dot solar concentrator (QDSC) comprises quantum dots (QDs) seeded in materials such as plastics and glasses that are suitable for incorporation into building façades. Photovoltaic (PV) cells attached to the edges convert direct and diffuse solar energy collected into electricity for use in the building. Small scale QDSC devices were fabricated. Devices have been characterised to determine current, voltage and power readings. Electrical conversion efficiencies, fill factors and comparative concentrating factors are reported.     

The potential applications and advantages of powering solar air-conditioning systems using concentrator augmented solar collectors

Non-concentrated evacuated tube heat pipe solar collectors have been reported to show higher fluid temperatures with improved thermal performance in the low to medium temperature range (?60 °C) due to low heat losses but suffer higher heat losses at the medium to higher temperature range (?80 °C) which reduces their efficiency compared to concentrated evacuated tube heat pipe solar collectors. To operate as stand-alone systems capable of attaining temperatures in the range of 70-120 °C, an innovative concentrator augmented solar collector can be an attractive option. The performance of a combined low-concentrator augmented solar collector in an array of evacuated tube heat pipe solar collectors defined as concentrator augmented evacuated tube heat pipe array (CAETHPA) and an array of evacuated tube heat pipe collectors (ETHPC) were tested and compared and results presented in this paper. The analysis of the experimental data allows concluding that the use of a CAETHPA is a more efficient alternative for integrating renewable energy into buildings with higher fluid temperature response, energy collection and lower heat loss coefficient compared to the use of evacuated tube heat pipe collector array (ETHPA).     

A solar flux density calculation for a solar tower concentrator using a two-dimensional hermite function expansion

The calculation of flux density on the central receiver due to a large number of flat polygonal reflectors having various orientations is a basic part of the system simulation problem for the tower concept of solar energy collection. A two-dimensional Hermite function expansion is adapted to the simulation problem, and numerical results are contrasted with an analytic integration of the solar flux density at specific nodes on an image plane. Various measures of error in the flux density calculation are monitored vs distance to the image plane and orientation of the reflector. The flux densities predicted by the statistical method compare favorably with those of the analytic model and require approximately one-tenth the computer time.     

Thermal performance of a linear Fresnel reflector solar concentrator using a black liquid

Results of some simple experiments conducted on a Linear Fresnel reflector solar concentrator using a tubular transparent corning glass absorber through which a highly absorbent black liquid is passed are presented in this short communication.     

Pilot production of concentrator silicon solar cells: Approaching industrialization

Using a simple process, high-efficiency silicon concentrator solar cells have proved to achieve up to 21% efficiency at 100×. The purpose of this work is to prove the feasibility of their industrialisation by setting up a pilot line and manufacturing a significant number of cells for a 100× concentrator system. The process has been successfully verified by modifying the antireflection coating, the annealing process and the back contact. This yielded an average efficiency of 18.5% at 100× with 70% of cells having an efficiency >18% and costs ranging from 0.31 to 0.41 €/W. A fast learning curve is shown which suggests optimistic results indeed for further industrialisation.     

The optimisation of a solar timber drier using an adsorbent energy store

This paper is concerned with determining the optimised design of a solar timber drier equipped with an adsorbent energy store. To perform the optimisations, use has been made of a COMPLEX search method, each point being generated using a simulation programme describing the drier. For the cost data used, the adsorbent store gave lower total costs than did a gravel bed store, and the best solar plus auxiliary energy system was cheaper than a conventionally heated system when the energy used was electricity.     

Optical flux mapping of a seasonally adjusted linear solar concentrator using a triangular absorber

A calibrated solar cell was used as a sensor to measure the flux distribution over a triangular absorber of a seasonally adjusted linear solar concentrator. The sensor was used to measure the flux distribution along the sides of the triangular absorber. The results are presented graphically and discussed.     

Cost-effective solar furnace system using fixed geometry Non-Imaging Focusing Heliostat and secondary parabolic concentrator

A novel cost-effective solar furnace system is proposed to be consisted of a Non-Imaging Focusing Heliostat (NIFH) and a much smaller parabolic concentrator. In order to simplify the design and hence leading to the cost reduction, a fixed geometry of the NIFH heliostat is adopted in the novel solar furnace system by omitting the requirement of continuous astigmatic correction throughout the year with the use of local controllers. The performance of this novel solar furnace configuration can be optimized when the heliostat’s spinning-axis is orientated in such a way that the annual variations of incident angle and therefore the annual variations of aberrant image size are the least. To verify the new configuration, a prototype solar furnace has been constructed at Universiti Tunku Abdul Rahman.     

High temperature solar collector with optimal concentration: Non-focusing fresnel lens with secondary concentrator

A non-evacuated collector consisting of a linear Fresnel lens and a second stage concentrator of the CPC type is described and tested in detail. Use of a Fresnel lens accomplishes two different objectives simultaneously: it allows for the design of a nearly ideal light collector (of the CPC type) of high concentration and height-to-aperture ratio close to 1 and plays the role of a cover, making the collector less sensitive to the environment than one with exposed reflector surface. The geometric concentration is 15.56 and the acceptance half angle is 3°. The optical efficiency measured with an Active Cavity Radiometer (ACR) is 65.6 per cent and the efficiency at of 0.235 is 48 per cent (ΔT = T_avfluid − T_amb = 200°C, I_ACR = 850 W/m²). Heat loss measurements for double glazed configurations are reported and the resulting efficiency at of 0.3 is predicted to be 48 per cent. These numbers are expected to be raised by 3 percentage points for a next generation of lenses. The collector is mounted with its tracking axis oriented oriented NS since EW tracking axis orientation is impractical for a linear Fresnel lens, but its wide acceptance angle permits tracking by a simple clock mechanism at constant speed. Two different strategies are considered (i) polar mount, (ii) two adjustments of the tracking axis a year (summer and winter); the predicted yearly performance is calculated for four locations and four working fluid temperatures.The projected cost is estimated to be $70.00/m² (1976 dollars), possible because the construction of the collector lends itself to the use of inexpensive materials such as plastic and glass.     

Error diagnosis and optimisation of c-Si solar cell processing using contact resistances determined with the Corescanner

The screen-printing metallisation process used for 90% of industrially produced solar cells is sensitive to process conditions. The best way to monitor this contacting process is to measure contact resistance. The standard technique used is the transmission line method. However, this only measures contact resistance locally.In this paper, contact resistance over the whole cell surface is measured with the newly developed ‘Corescanner’. Using this instrument, the relation between processing parameters and solar cell contact resistance distribution is investigated. Our most important finding is that poor contacting results in large inhomogeneities in contact resistance. Even for cells with very low fill factors, regions of low contact resistance can be found.To conclude, the Corescanner provides us with a technique to monitor contact resistance. This instrument is an invaluable tool for fault detection, error diagnosis and process optimisation.     

Concentration characteristics of a two stage solar concentrator: Effect of primary mirror surface errors

The concentration characteristics of a two-stage linear solar concentrator employing a perfectly tracked parabolic trough as the primary and a seasonally adjusted compound parabolic concentrator as the secondary stage with a flat horizontal absorber are studied. The Monte Carlo ray trace technique is used for this purpose. The effect of randomly distributed primary mirror surface errors on the concentration characteristics of the two stage concentrator is also investigated.     

Cost comparative study for new water distillation techniques by solar energy using

The aim of this work is to compare the extra added cost of different new water distillation techniques to the cost of extra collected distilled water. The comparison is between; a traditional single slope solar still, a solar still with installed reflecting mirror, a solar still with replacing the flat water with step wise basin, a solar still with a single axis sun tracking system and a solar still with sun tracking system and replacing the flat water basin with step wise basin. It was found that all cases of modifications have an economical effect except one which is the system with sun tracking. The most feasible system is the step wise design, which gave the shortest pay back period of 2.1 years, followed by the system with sun tracking and step wise, which gave the payback period of 3.6 years, followed by the system with reflecting mirrors, which gave the payback period of 4.6 years.